Non destructive process for ascertaining the tensile strength of grey iron castings



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R. ZIEGLER EIAI. 3,003,351

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51mm}! 0? GREY IRON CASTIIGS Filed Aug. 12, 1%?

o MEAN VALUE FROM 2 MEASUREMENTS 0 SINGLE MEASUREMENT Veg .74 -5 19 oao132v? WENTORS ROLF ZIEGLER RICHARD GERSTNER flunk, 9M 3% A TTORNEYSUnited States Patent Ofiice 3,003,351 Patented Oct. 10, 1961 NONDESTRUCTIVE PROCESS FOR ASCERTAIN- ING THE TENSILE STRENGTH F GREY IRONCASTINGS Rolf Ziegler and Richard Gerstner, both of Parkstrasse 21,

Leoben, Styria, Austria Filed Aug. 12, 1957, Ser. No. 677,421

- 3 Claims. (Cl. 73-675) Tensile strength is one of the most importantproperties of castings as it is the basis for designing the dimensionsof the same. In the case of grey iron castings the tensile strength doesnot only depend on the chemical composition but also largely on the wallthickness and the cooling conditions. It is therefore extremelyditdcult' to ascertain the tensile strength of grey iron castings atcertain areas of the respective pieces.

For verifying the tensile strength of grey iron castings it is thereforenot suflicient in general to ascertain the strength on separately casttest bars but it would be necessary to cut out from a casting itself atthe area where the strength is to be measured a test bar which can beruptured by well known test bar pulling devices. This method iscomplicated, costly and time-wasting as it causes considerable expensein the workshop and besides entails destruction of the casting.

Colland has developed a new process for finding out the tensile strengthof a casting in an indirect manner. According to the Colland process atest bar of about 30 mm. in diameter is cast out of the same charge fromwhich the casting is to be made. This test bar is used to ascertain thetensile strength of the casting by standard pulling methods. Because ofthe varying wall thickness of grey iron castings the value obtained onthe test bar is not identical with that of the casting itself. Collandrequires that in addition the Brinell hardness of the casting area andthe test bar be ascertained. The tensile strength at the respective areaof the casting may then be calculated according to the followingformula:

TS =tensile strength at the area in question of the casting.

TS =tensile strength of the separately cast test bar.

HB =Brinell hardness of the casting at the point in question.

HB =Brinell hardness of the test bar separately cast.

The advantage of this method is that the casting need not be destroyed.It presupposes however that the casting is perfectly faultless at thearea in question and that it does not have at such area segregations,piping, blisters and other faults due to inhomogeneousness. Besides thismethod is also intricate and time-wasting. Finally it is diflicult insome cases to measure the Brinell hardness on the castings themselvesunless special presses are available for this purpose.

The evaluation of the tensile strength of the casting from the chemicalcomposition and the wall thickness by means of suitable graphs can onlyvyield approximate figures and will never provide exact measurements.

The uncertainty of the true tensile strength of castings due to thedifficulties described above results in castings which are in many casesdesigned overstrength, thus increasing the costs considerably andunnecessarily.

The present invention provides a simple and exact oper. ating method forascertaining tensile strength of grey iron castings- In the singlefigure of the drawing there is shown a graph on the horizontal leg ofwhich is shown the projection of results obtained from first measuringthe actual thickness of a plurality of test bars by mechanical means anddividing such thickness by the apparent thickness as indicated by thespeed of an ultra-sonic impulse. The vertical leg of the graph is aprojection of the tensile strength of the same test bars as determinedby pulling them in a standard tensile testing machine. The straight lineindicates the relationship between the variables on the horizontal andvertical lines of the graph.

According to the invention it has been found that there exists a certainrelationship between the speed of ultrasonic impulses through grey ironcastings and the tensile strength existing at the same area of thecasting. Therefore the determination of the speed of the ultra-sonicimpulses in the part of the casting being tested for tensile strengthcan be easily read from an empirically arranged graph which will havebeen established for solidified grey iron castings whose metal wasmelted in the same type of melting equipment. The invention willdetermine the tensile strength values ranging between 12 and 32kilograms per square millimeter.

It is not absolutely necessary to measure the absolute value of theultra-sonic impulses. Usually it is sufiicient to ascertain simply therelationship between the sound speed of the ultra-sonic impulses throughthe casting to be tested and the speed of the ultra-sonic impulses insteel.

By way of example of such a process for determining the tensile strengthof grey iron castings there is now described in detail a method whichhas been developed on the basis of a great number of experiments.

The invention employs a standard ultra-sonic impulse instrument, such asa Sperry refiectoscope, which is usually employed for ascertaining flawsin metals. This instrument is additionally provided with a device formeasuring the wall thickness of metal and which has been calibrated tothe speed of ultrasonic impulses in steel. The speed of ultrasonicimpulses in steel is used as a constant.

By means of this device for measuring the wall thickness the wallthickness of the grey iron casting under investigation is ascertained atthe area selected. However the thickness so determined does not agreewith the actual wall thickness of a grey iron casting at the selectedarea as the instrument used is calibrated for steel whereas grey ironcastings have a different ultra-sonic impulse speed. The wall thicknessascertained in this manner is therefore designated as apparent wallthickness. At the same time the real wall thickness of the area inquestion of the grey iron castings is determined mechanically forinstance by means of a calipers. The relationship of the speed ofultra-sonic impulses in the grey iron casting being tested and the speedof ultra-sonic impulses in steel equals the relationship between theactual thickness of such grey iron casting as determined by calipers orother mechanical means and the apparent thickness of such casting asdetermined by the speed of the ultrasonic impulses in such casting asmeasured by an instrument calibrated for the speed of ultra-sonicimpulses in steel. This relationship may be expressed by the followingformula:

Gg real s:

apparent.

51: whereupon at the same areas test bars were cut out and 3 theirtensile strength measured. The test bars used in this connection had thefollowing chemical composition:

The tensile strength of cast iron does not only depend on the carboncontent but also on the content of the silicon and the phosphorus. Theinfluence of these three elements is ascertained by way of the so-calledsaturation or carbon equivalent value which is computed according tothe'following formula:

We have used the above formula wherein S stands for the saturation valuefor calculating the saturation values of the given bars as follows:

Bar No.: 7 Saturation value 1103 0.81 1104 0.88 1105 0.89 .1106 0.93.1158 0.92 1176 0.97 1196 1.05 1197 1.00'

shown in the drawing. The relation of the tensile strength 7 to therelative speed of the ultra-sonic impulse can be shown as a straightline as indicated on the graph. This is thenused as a determination forfuture measurements of tensile strength. The straight line correspondsto the following equation:

' TS :tensile strength in lip/mm.

j =.relative speed of ultra-sonic impulses as described "at b 7 Thedeviation of the individual points from the straight line drawn is foundto be reasonable and results from segr'egations and other faults ofhomogeneity deficiency The test bars used for this purpose were producedunder utterly difierent conditions from those which had been employedfor establishing the straight line of the graph. For example, some ofthe test bars were cast in shell moulds and the following table showsthe relationship of the tensiles obtained:

Tensile Strength Tensile Strength measured by ascertained by Deviations,means of rupture means of ultralip/mm. test, kp./nm1.= sound implement,a

' kpJmrn.

It appears that the deviations are within the domain of i2kp./mm.

Various embodiments of the invention may be employed within the scope ofthe accompanying claims.

What is claimed is I 1. A non-destructive testing method for determiningthe tensile strength of a grey iron casting, which comprises the stepsof subjecting a plurality of grey iron test castings of varying chemicalcomposition to ultrasonic impulses induced by a machine calibrated forsteel to determine the apparent thickness of each of said castings,rupturing each of said test castings to determine the tensile strengthof each, producing a graph by plotting the quotient obtained by dividingthe actual thickness of each of said test castings as determined bymechanical means by the respective apparent thickness of said castingsagainst thetensile strength thereof and thereafter drawing asubstantially straight line through the plotted points, and subjecting agrey iron casting of unknown tensile strength to ultrasonic impulsesinducedby a machine calibrated for steel to determine the apparentthickness of said casting, said apparent thickness being divided intothe actual thickness of said casting and the resulting quotient locatedon said graph to establish the tensile strength of said casting.

2. A non-destructive method for determining the tensile strength of agrey iron casting, which comprises the steps of subjecting a series ofgrey iron test specimens of varying chemical composition and of knownWall thicknesses to ultrasonic impuises to determine the velocity ofpropagation of said impulses through each of said specimens, rupturingeach of said grey iron test speci mens to ascertain the tensile strengththereof, producing a graph by plotting the quotients obtained bydividing the velocity of propagation of said impulses through each ofsaid grey iron test specimens by the velocity of propagation ofultrasonic impulses through a steel test specimen having an actual wallthickness equal to that of the correspinding grey iron 'test specimensagainst the tensile strength of the respective grey iron test specimensand in the material. It is known that the results of rupturing testsoriginating from the same charge and being cast under the sameconditions are subject to an error of about plus or minus. 2 kp/mmfi. Inthe case of the normal means of rupturing test bars irregularities havea greater effect as the volume on which the determination is based isvery small. The measurements of the ultra-sonic instrument cover a muchgreater volume as the probe is large and covers a large area as comparedto the wall thickness and the margin of error is reduced.

The straight line of the graph applies to cast ironof standard texture.But cast iron of a difierent textural formation, for instance, thatwitha different graphite arrangement, can be investigated by this method,although a separate graph would have to be developed.

thereafter drawing a substantially straight line through the plottedpoints, and subjecting a grey iron casting of unknown tensile strengthto ultrasonic impulses to' determine the velocity of propagation of saidimpulses therethrough, said velocity of propagation of said impulsesthen being divided by the velocity of propagation of ultrasonic impulsesthrough a steel test specimen of the same actual wall thickness and the"quotient obtained being used in connection with said graph to establishthe tensile strength of said grey iron casting.

'3. -A non-destructive testing method for determining the tensilestrength of a grey iron casting in any given area, which comprises thesteps of subjecting a series of grey iron test castings of varyingchemical composition to ultrasonic impulses induced by a machinecalibrated to a constant to determine the apparent thickness of each ofsaid castings, rupturing each of said grey iron test castingsto-determinethe tensile strength of'each, producing an empi icalrelationship between the tensile strength of said test castings and thequotient obtained by dividing the actual thickness of each of said testcastings as determined by mechanical means by the apparent thickness ofsaid castings, and subjecting a given area of a grey iron casting ofunknown tensile strength to ultrasonic impulses induced by a machinecalibrated to a constant to determine the apparent thickness of saidarea of said casting, the actual thickness of Said area then beingdivided by the apparent thickness and the resultant quotient employed inthe previously determined empirical relationship to determine thetensile strength of the said area of the casting.

References Cited in the file of this patent UNITED STATES PATENTS GunnFeb. 15, 1949 Arnold Sept. 16, 1958

3. A NON-DESTRUCTIVE TESTING METHOD FOR DETERMINING THE TENSILE STRENGTHOF A GREY IRON CASTING IN ANY GIVEN AREA, WHICH COMPRISES THE STEPS OFSUBJECTING A SERIES OF GREY IRON TEST CASTINGS OF VARYING CHEMICALCOMPOSITION TO ULTRASONIC IMPULSES INDUCED BY A MACHINE CALIBRATED TO ACONSTANT TO DETERMINE THE APPARENT THICKNESS OF EACH OF SAID CASTINGS,RUPTURING EACH OF SAID GREY IRON TEST CASTING TO DETERMINE THE TENSILESTRENGTH OF EACH, PRODUCING AN EMPIRICAL RELATIONSHIP BETWEEN THETENSILE STRENGTH OF SAID TEST CASTINGS AND THE QUOTIENT OBTAINED BYDIVIDING THE ACTUAL THICKNESS OF EACH OF SAID TEST CASTINGS ASDETERMINED BY MECHANICAL MEANS BY THE APPARENT THICKNESS OF SAIDCASTINGS, AND SUBJECTING A GIVEN AREA OF A GREY IRON INDUCED BY AMACHINE CALIBRATED TO A CONSTANT TO DETERMINE THE APPARENT THICKNESS OFSAID AREA OF SAID CASTING, THE ACTUAL THICKNESS OF SAID AREA THEN BEINGDIVIDED BY THE APPARENT THICKNESS AND THE RESULTANT QUOTIENT EMPLOYED INTHE PREVIOUSLY DETERMINED EMPIRICAL RELATIONSHIP TO DETERMINE THETENSILE STRENGTH OF THE SAID AREA OF THE CASTING.